Understanding the erosion of freshwater biodiversity has become a global imperative, but consistent series of long-term data from which to appraise changes are rare. In central Wales (UK), the Lynn Brianne Stream Observatory has provided unique insight into the complexity of biodiversity dynamics over four decades, revealing how apparent stasis in alpha- and beta-diversity might mask non-random functional changes in macroinvertebrate assemblages. Assessments of synchrony and stability at population and community levels reveal the effect of climatic variations in which warmer, wetter phases of the North Atlantic Oscillation (NAO) have been associated with large interannual changes in community composition. Moreover, these positive NAO periods have brought greater synchrony in species abundances within streams (community synchrony) and across streams (spatial population synchrony). Increasing synchrony can destabilise ecosystems with consequences for the persistence of populations. Preliminary analyses at Lynn Brianne suggest that species with greater spatial synchrony tend to decline in abundance over time. For instance, the abundance of cold-adapted species has declined by 40% since the 1980s reflecting the general increase in temperatures. Moreover, populations of these species displayed significantly higher spatial synchrony than warm-adapted species, which increased by 30% over the same time period. We suggest that both directional climate warming and the NAO contribute to the long-term reorganisation of benthic communities in temperate headwater
